NED Abstract

Copyright by American Astronomical Society.
Reproduced by permission
2000ApJ...536..645C
Diffuse Ionized Gas in a Sample of Edge-on Galaxies and Comparisons with HI
and Radio Continuum Emission
Joseph A. Collins, Richard J. Rand, and Nebojsa Duric
University of New Mexico, Department of Physics and Astronomy, 800 Yale
Boulevard NE, Albuquerque, NM 87131
and
Reni A. M. Walterbos
New Mexico State University, Department of Astronomy, MSC4500, Box 30001,
Las Cruces, NM 88003
Received 1999 September 29; accepted 2000 January 7
ABSTRACT
We present H{alpha} images of four edge-on galaxies: NGC 5775, NGC 3044,
NGC 4183, and NGC 3556. Our goals are twofold: first, to continue the study
of the occurrence and physical properties of extraplanar diffuse ionized
gas (DIG), and second, to compare extraplanar H{alpha}, H I, and radio
continuum emission. The DIG of NGC 5775 is found to be concentrated in
numerous bright filaments extending as high as 6 kpc off the midplane. NGC
3044, on the other hand, has a bright, smooth region of DIG above the
central part of the disk with some faint filaments at higher galactocentric
radius. Vertical profile fits to the extraplanar emission show that both
NGC 5775 and NGC 3044 have DIG layers more extended than the Reynolds
layer. In contrast, NGC 4183 shows very little extraplanar DIG. Profile
fits suggest that it is very unlikely that the DIG layer of NGC 4183 is as
extended as the Reynolds layer. The correlation between levels of star
formation and prominence of DIG layers for these galaxies is consistent
with previous observations. A prediction of the "chimney model" of the ISM
is that extraplanar H{alpha} features should have H I and radio continuum
counterparts. Comparisons among these three tracers have revealed, for the
first time, correlations on the scale of individual extraplanar filaments
in a galactic disk. NGC 5775 shows correlations between H{alpha} filaments,
H I shells, and radio continuum spurs that are consistent with a chimney
model. However, features identified as possible chimneys in NGC 5775 show
radio spectral indices steeper than would be expected if cosmic rays were
swept into the halo via galactic winds. Calculations of cosmic-ray
propagation scale lengths within the DIG filaments demonstrate that
diffusion may indeed be responsible for transporting cosmic rays through
chimneys marked by DIG filaments. Correlated emission in these three
tracers is also seen in NGC 3044, where extraplanar H I and radio continuum
features appear to be associated with the galaxy's central region of
extraplanar DIG, as well as filaments above the disk. Such correlations are
not as prominent in NGC 3556, where extraplanar DIG detection is hampered
by the galaxy's relatively low inclination.
Subject headings: diffuse radiation-galaxies: ISM-galaxies: spiral-stars:
formation